For well over 75 years the internal combustion engine has been mankind's primary source of motive power. It would be difficult to overstate its importance or the engineering effort expended in seeking its perfection. So mature and well understood is the art of internal combustion engine design that most so called “new” engine designs are merely designs made up of choices among a variety of known alternatives. For example, an improved output torque curve can easily be achieved by sacrificing engine fuel economy. Emissions abatement or improved reliability can also be achieved with an increase in cost. Still other objectives can be achieved such as increased power and reduced size and/or weight but normally at a sacrifice of both fuel efficiency and low cost.
The challenge to contemporary designers has been significantly increased by the need to respond to governmentally mandated emissions abatement standards while maintaining or improving fuel efficiency. In view of the mature nature of engine design, it is extremely difficult to extract both improved engine performance and emissions abatement from further innovations of the basic engine designs commercially available today. Yet the need for such innovations has never been greater in view of the series of escalating emissions standards mandated for the future by the United States government and other countries. Attempts to meet these standards include some designers looking for a completely new engine design.
Traditionally, there have been two primary forms of reciprocating piston or rotary internal combustion engines: diesel and spark ignition engines. While these engine types have similar architecture and mechanical workings, each has distinct operating properties which are vastly different from each other. Diesel and spark ignited engines effectively control the start of combustion (SOC) using simple, yet distinct means. The diesel engine controls the SOC by the timing of fuel injection. In a spark ignited engine, the SOC is controlled by the spark timing. As a result, there are important differences in the advantages and disadvantages of diesel and spark-ignited engines. The major advantage that a spark-ignited natural gas, or gasoline, engine has over a diesel engine is the ability to achieve extremely low NOx and particulate emissions levels. The major advantage that diesel engines have over premixed charge spark ignited engines (such as passenger car gasoline engines and lean burn natural gas engines) is higher thermal efficiency. One key reason for the higher efficiency of diesel engines is the ability to use higher compression ratios than premixed charge spark ignited engines (the compression ratio in premixed charge spark ignited engines has to be kept relatively low to avoid knock). A second key reason for the higher efficiency of diesel engines lies in the ability to control the diesel engine's power output without a throttle. This eliminates the throttling losses of premixed charge spark ignited engines and results in significantly higher efficiency at part load for diesel engines. Typical diesel engines, however, cannot achieve the very low NOx and particulate emissions levels which are possible with premixed charge spark ignited engines. Due to the mixing controlled nature of diesel combustion a large fraction of the fuel exists at a very fuel rich equivalence ratio which is known to lead to particulate emissions. Premixed charge spark ignited engines, on the other hand, have nearly homogeneous air fuel mixtures which tend to be either lean or close to stoichiometric, resulting in very low particulate emissions. Another consideration is that the mixing controlled combustion in diesel engines occurs when the fuel and air exist at a near stoichiometric equivalence ratio which leads to high temperatures. The high temperatures, in turn, cause high NOx emissions. Lean burn premixed charge spark ignited engines, on the other hand, burn their fuel at much leaner equivalence ratios which results in significantly lower temperatures leading to much lower NOx emissions. Stoichiometric premixed charge spark ignited engines, on the other hand, have high NOx emissions due to the high flame temperatures resulting from stoichiometric combustion. However, the virtually oxygen free exhaust allows the NOx emissions to be reduced to very low levels with a three-way catalyst.
Relatively recently, some engine designers have directed their efforts to another type of engine which utilizes premixed charge compression ignition (PCCI) or homogeneous charge compression ignition (HCCI), hereinafter collectively referred to as PCCI. Engines operating on PCCI principles rely on autoignition of a relatively well premixed fuel/air mixture to initiate combustion. Importantly, the fuel and air are mixed upstream of the cylinder, e.g., in the intake port, or in the cylinder, long before ignition occurs. The extent of the mixture may be varied depending on the combustion characteristics desired. Some engines arc designed and/or operated to ensure the fuel and air are mixed into a homogeneous, or nearly homogeneous, state. Also, an engine may be specifically designed and/or operated to create a somewhat less homogeneous charge having a small degree of stratification. In both instances, the mixture exists in a premixed state well before ignition occurs and is compressed until the mixture autoignites. Thus, PCCI combustion is characterized in that: 1) the vast majority of the fuel is sufficiently premixed with the air to form a combustible mixture throughout the charge by the time of ignition; and 2) ignition, that is, the very onset or start of combustion, is initiated by compression ignition. Unlike a diesel engine, the timing of the fuel delivery, for example the timing of injection, in a PCCI engine does not strongly affect the timing of ignition. Preferably, PCCI combustion is characterized in that most of the mixture is significantly leaner than stoichiometric to advantageously reduce emissions, unlike the typical diesel engine cycle in which a large portion, or all, of the mixture exists in a rich state during combustion.
Because an engine operating on PCCI combustion principles has the potential for providing the excellent fuel economy of the diesel engine while providing NOx and particulate emissions levels that are much lower than that of current spark-ignited engine, it has also recently been the subject of extensive research and development. U.S. Pat. Nos. 4,768,481; 5,535,716; and 5,832,880 all disclose engines and methods for controlling PCCI combustion in engines. Researchers have used various other names in referencing PCCI combustion including homogeneous charge compression ignition (HCCI) as well as others such as “ATAC” which stands for “Active Thermo-Atmosphere Combustion.” (SAE Technical Paper No. 790501, Feb. 26-Mar. 2, 1979), “TS” which stands for “Toyota-Soken” (SAE Technical Paper No. 790840, Sep. 10-13, 1979), and “CIHC” which stands for “compression-ignited homogeneous charge” (SAE Paper No. 830264, 1983). All of these terms are hereinafter collectively referred to as PCCI.
Although PCCI combustion may result in improved fuel economy and substantially reduced emissions, it is difficult for an engine to operate in a PCCI mode over a wide range of operating conditions, ranging from cold start-up to various levels of engine load. For example, SAE Technical Paper No. 790501 reports that PCCI combustion (ATAC) could be made to occur in a two-stroke engine at low load over a wide speed range. To attain PCCI combustion, the following conditions were found to be important. The quantity of mixture and the air/fuel ratio supplied to the cylinder must be uniform from cycle to cycle. The scavenging “directivity” and velocity must have cyclic regularity to ensure the correct condition of the residual gases remaining in the cylinder. The temperature of the combustion chamber walls must be suitable. The scavenging passage inlet must be located at the bottom of the crankcase. It was found that at very light loads, PCCI was not successful because charge temperatures were too low. At very high loads, PCCI was not successful because the residual gas quantity was too low. In between these regions, PCCI combustion was successful.
As a result, research has been directed to an engine capable of operating in multiple combustion modes. For example, SAE Technical Paper No. 892068, entitled “Homogeneous-Charge Compression Ignition (HCCI) Engines”, Thring, R., Sep. 25, 1989, investigated PCCI operation of a four-stroke engine. The paper suggests an engine that would operate in a conventional spark-ignition mode at start-up and at high loads, but in a PCCI mode at part-load and idle. Others have produced two-stroke motorcycle engines which successfully use a spark to initiate combustion upon starting the engine, at the lowest load conditions, such as idling, and at high loads while operating in a PCCI mode during a low to mid-load range. The change-over between spark-ignition and PCCI modes is controlled by an electronic control unit. SAE papers 920512 and 972874 are noted for disclosing experimental results comparing PCCI combustion to spark-ignition combustion, but fail to specifically teach the manner in which transitions between modes of operation could be most effectively achieved. German Patent No. 198 18 596 also discloses a process of operating an engine in a PCCI mode at least low loads and in a spark-ignition mode at high loads.
Other efforts have focused on the combination of a diesel combustion operating mode and a PCCI mode. For example, SAE paper No. 971676 entitled “Homogeneous Charge Compression Ignition (HCCI) of Diesel Fuel” reports test results of an engine which includes starting in a diesel combustion mode and, once the temperature of the engine stabilized, configuring the engine to a PCCI mode. U.S. Pat. No. 5,875,743 discloses an engine which operates in a diesel combustion mode in response to engine operating parameters indicative of engine speed and load values within a first predefined range, and in a PCCI mode in response to engine operating parameters indicative of engine speed and load within a second predefined range. Generally, the engine appears to operate in a diesel mode during light and heavy loads and in a PCCI mode at other conditions. A look-up table may be used to define the speed and load ranges at which the engine will run in conventional diesel mode, and the speeds and load ranges the engine will switch to the PCCI mode. If misfire or knock is detected, the PCCI mode can be adjusted or the engine switched back to the diesel mode. Transition between the diesel mode to the PCCI mode is primarily accomplished by switching between an in-cylinder fuel injector and a port injector for early injection and mixing of fuel, or varying the timing of injection of the in-cylinder injector.
Patent application Ser. No. 08/916,437 filed on Aug. 22, 1997 (published as International Patent Application No. PCT/US97/14815), currently assigned to the Assignee of the present invention, discloses an engine and method of operation which includes multiple combustion modes. The engine is switched between a conventional diesel mode and/or spark-ignited mode and a PCCI mode depending on the operating conditions of the engine.
Still, there is a need for an engine, and method of engine operation, which includes more effectively and more efficiently operating in, and transitioning between, a PCCI mode and one or both of a diesel mode and a spark-ignition mode.